The present invention relates to optical data storage devices as used in the computer industry and video recording industry, and in particular to an apparatus and method for recording a serial bit train of binary data on a recording medium of such recording devices.
The optical disk used to record information in an optical disk data storage device is in the form of a circular disk plate having a thin layer into which a plurality of reflectivity changes are formed on and along a plurality of circular tracks arranged concentrically or on and along a spiral track. Data in a computer system consists of a series of characters, for example, alphabetic characters, numbers, or special symbols, wherein each character is formed using seven or eight binary bits. To store these characters on the optical disk media, each of the seven or eight bits is written serially along the track, since each track is one bit wide. The process of storing data on a track of the optical storage media consists of arranging reflectivity or transmissivity changes along such track to represent binary ones and binary zeros. In its unrecorded state, a reflective disk medium is either highly reflective or non-reflective. One way to represent binary ones and zeros on such a surface is to allow the unchanged, that is, unrecorded, surface of the disk medium to represent a binary zero, and by means of a coherent radiation beam, such as a helium-neon laser, alter the reflectivity of the surface at a particular spot to represent a binary one. For example, if the unrecorded surface has a high reflectivity, in order to record a binary one bit the coherent radiation beam would be used to create an area of low reflectivity in every location where such a one bit is to be recorded. In its unrecorded state, a transmissive disk recording medium is either clear or opaque. For example, if the unrecorded clear state represents a binary zero, then a coherent radiation beam would be used to alter the medium from clear to opaque to represent a binary one.
In order to retrieve data recorded on the surface, a second coherent radiation beam, having insufficient power to alter the surface of the recording track, is directed toward the medium and a photo detector is used to detect radiation received from the medium. As the beam passes a given area, if radiation is received from the medium, indicating a reflective area on a reflective medium or a clear area on a transmissive medium, then a binary zero is recorded on the medium at that point, whereas if no radiation is received from the medium at such a point, a binary one is recorded thereon. This type of recording could also be reversed, such that a reflective or clear area represents a binary one, and a non-reflective or opaque area represents a binary zero.
One disadvantage to using this type of recording method is that the radiation beam must be turned on and off rapidly to record a series of ones and zeros at a high density. Because of write laser diode device limitations, these rapid on-off changes cause size variations in the reflectivity areas. Another disadvantage to this recording method is that a defect in the recording media will be detected as either a one or a zero and not as a defect. Another disadvantage to this system is that the reflected energy varies whenever a long series of binary ones or a long series of binary zeros is present. This variation in reflected energy complicates the tracking and focusing systems which must align the read beam along the track and focus the beam at the plane of the thin layer.
There is need in the art then for a method of recording data on an optical disk storage medium that does not require the coherent radiation beam that records the data on such medium to turn on and off. There is also a need in the art for a recording system that will simplify the focus and tracking requirements of the coherent radiation beam by providing a constant reflected energy level.